Electrophoretic fluid

ABSTRACT

The present invention is directed to a display fluid comprising charged composite pigment particles dispersed in a solvent. The composite pigment particles have a density which matches to the density of the solvent in which they are dispersed. A display fluid comprising the composite pigment particles provides improved display performance.

This application claims priority to U.S. Provisional Application No.61/439,302, filed Feb. 3, 2011; the content of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is directed to the preparation of compositepigment particles that can be used to form an electrophoretic fluid andthe resulting display fluid.

BACKGROUND OF THE INVENTION

An electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon influencing charged pigment particlesdispersed in a dielectric solvent. An EPD typically comprises a pair ofspaced-apart plate-like electrodes. At least one of the electrodeplates, typically on the viewing side, is transparent. Anelectrophoretic fluid composed of a dielectric solvent with chargedpigment particles dispersed therein is enclosed between the twoelectrode plates.

An electrophoretic fluid may have one type of charged pigment particlesdispersed in a solvent or solvent mixture of a contrasting color. Inthis case, when a voltage difference is imposed between the twoelectrode plates, the pigment particles migrate by attraction to theplate of polarity opposite that of the pigment particles. Thus, thecolor showing at the transparent plate may be either the color of thesolvent or the color of the pigment particles. Reversal of platepolarity will cause the particles to migrate back to the opposite plate,thereby reversing the color.

Alternatively, an electrophoretic fluid may have two types of pigmentparticles of contrasting colors and carrying opposite charges, and thetwo types of pigment particles are dispersed in a clear solvent orsolvent mixture. In this case, when a voltage difference is imposedbetween the two electrode plates, the two types of pigment particleswould move to the opposite ends (top or bottom) in a display cell. Thusone of the colors of the two types of the pigment particles would beseen at the viewing side of the display cell.

For all types of the electrophoretic displays, the fluid containedwithin the individual display cells of the display is undoubtedly one ofthe most crucial parts of the device. The composition of the fluiddetermines, to a large extent, the lifetime, contrast ratio, switchingrate and bistability of the device.

In an ideal fluid, the charged pigment particles remain separate and donot agglomerate or stick to each other or to the electrodes, under alloperating conditions. In addition, all components in the fluid must bechemically stable and compatible with the other materials present in anelectrophoretic display.

Currently, the pigment particles in an electrophoretic fluid often havea density which is much higher than that of the solvent in which theparticles are dispersed, thus causing performance issues, such as poorgrey level bistability, vertical driving and settling phenomena.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show the composite pigment particles of the presentinvention.

FIG. 2 shows reaction steps of a process suitable for the preparation ofthe composite pigment particles of the present invention.

SUMMARY OF THE INVENTION

The present invention is directed to a display fluid comprising chargedcomposite pigment particles dispersed in a solvent, wherein each of saidcomposite pigment particles comprises at least a core pigment particle,a shell coated over the core pigment particle and steric stabilizermolecules on the surface of the composite pigment particles.

In one embodiment, the density of the composite pigment particlessubstantially matches to that of the solvent.

In one embodiment, the difference between the density of the compositepigment particles and the density of the solvent is less than 2 g/cm³.

In one embodiment, the core pigment particle is an inorganic pigmentparticle. In one embodiment, the core pigment particles are surfacetreated.

In one embodiment, the shell is formed from an inorganic material. Forexample, the shell may be formed from silica, aluminum oxide, zinc oxideor a combination thereof. In one embodiment, the shell is porous if itis formed from an inorganic material. In one embodiment, the organiccontent of the composite pigment particles is in the range of about 10%to about 50% by weight, preferably more than about 15% up to about 30%by weight.

In one embodiment, the shell is formed from an organic material. Forexample, the shell may be formed from polyacrylate, polyurethane,polyurea, polyester or polysiloxane. In one embodiment, the organiccontent of the composite pigment particles is at least about 20% byweight, preferably about 20% to about 70% by weight and more preferablyabout 20% to about 40% by weight.

In one embodiment, the shell is completely incompatible or relativelyincompatible with the solvent.

In one embodiment, the steric stabilizer molecules are formed frompolyacrylate, polyethylene, polypropylene, polyester, polysiloxane or amixture thereof.

In one embodiment, the surface of the shell comprises functional groupsto enable charge generation or interaction with a charge control agent.

In one embodiment, the fluid further comprises a second type of chargedpigment particles. In one embodiment, the second type of charged pigmentparticles is composite pigment particles comprising at least a corepigment particle, a shell coated over the core pigment particle andsteric stabilizer molecules on the surface of the composite pigmentparticles. The two types of composite pigment particles in the fluid areof contrasting colors.

In one embodiment, the solvent in which the composite pigment particlesare dispersed is a hydrocarbon solvent or a mixture of a hydrocarbonsolvent and another solvent, such as a halogenated solvent or a siliconeoil type solvent.

In one embodiment, the composite pigment particles are prepared bydispersion polymerization or living radical polymerization.

DETAILED DESCRIPTION OF THE INVENTION

The first aspect of the present invention is directed to the compositepigment particles, as shown in FIGS. 1a and 1b . The composite pigmentparticles are closely density matched to a solvent in which they aredispersed, especially in a hydrocarbon solvent.

The composite pigment particles (10) may have one or more core pigmentparticles (11). The core particle(s) (11) is/are coated with a shell(12). There are steric stabilizer molecules (13) on the surface of thecomposite pigment particles.

The core pigment particles are preferably inorganic, such as TiO₂,BaSO₄, ZnO, metal oxides, manganese ferrite black spinel, copperchromite black spinel, carbon black or zinc sulfide pigment particles.They may be black, white or of another color.

The core particles may be optionally surface treated. The surfacetreatment would improve compatibility of the core pigment particles tothe monomer in a reaction medium or chemical bonding with the monomer,in forming the composite pigment particles. As an example, the surfacetreatment may be carried out with an organic silane having functionalgroups, such as acrylate, vinyl, —NH₂, —NCO, —OH or the like. Thesefunctional groups may undergo chemical reaction with the monomers.

The shell may be formed from an inorganic or organic material.

Inorganic shell materials may include silica, aluminum oxide, zinc oxideand the like or a combination thereof. Sodium silicate ortetraethoxysilane may be used as a common precursor for silica coating.

An organic shell may be formed from an organic polymer, such aspolyacrylate, polyurethane, polyurea, polyethylene, polyester,polysiloxane or the like. For example, a polyacrylate shell may beformed from monomer, such as styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylateor the like. A polyurethane shell may be formed from monomer oroligomer, such as multifunctional isocyanate or thioisocyanate, primaryalcohol or the like. A polyurea shell may be formed from monomercontaining reactive groups, such as amine/isocyanate,amine/thioisocyanate or the like. A person skilled in the art would beable to select proper monomer or oligomer and its variations, based onthe main idea of the present invention.

If the shell is inorganic, the structure of the shell may be porous toreduce density. The “organic content” of the resulting composite pigmentparticles would be in the range of about 10% to about 50% by weight,preferably more than about 15% up to about 30% by weight.

If the shell is organic, the “organic content” of the resultingcomposite pigment particles would be at least about 20% by weight,preferably about 20% to about 70% by weight and more preferably about20% to about 40% by weight.

The term “organic content” is determined by the total weight of theshell (12) and the steric stabilizers (13) divided by the total weightof the core pigment particles (11), the shell (12) and the stericstabilizers (13).

The density of the resulting shell, in any case, is preferably low,lower than 2 g/cm³ and more preferably about 1 g/cm³. The shellthickness may be controlled, based on the density of the shell materialand the desired final particle density.

The shell material is either completely incompatible or relativelyincompatible with the display fluid in which the composite pigmentparticles are dispersed. In other words, no more than about 5%,preferably no more than about 1%, of the shell material is miscible withthe display fluid.

In order to achieve this complete or relative incompatibility, the shellpolymer material may have polar functionality on its main chain or aside chain. Examples of such polar functionality may include —COOH, —OH,NH₂, R—O—R, R—NH—R and the like (wherein R is an alkyl or aryl group).Each of the side chains, in this case, preferably has less than 6 carbonatoms. In one embodiment, the main chain or the side chain may containan aromatic moiety.

In addition, the core pigment particle(s) and the shell should behave asone single unit. This may be achieved by cross-linking or anencapsulation technique, as described below.

The steric stabilizer (13) in FIG. 1 is usually formed of high molecularweight polymers, such as polyethylene, polypropylene, polyester,polysiloxane or a mixture thereof. The steric stabilizer facilitates andstabilizes the dispersion of the composite pigment particles in asolvent.

Furthermore, the surface of the shell may optionally have functionalgroups that would enable charge generation or interaction with a chargecontrol agent.

The second aspect of the present invention is directed to thepreparation of the composite pigment particles of the present invention,which may involve a variety of techniques.

For example, they may be formed by dispersion polymerization. Duringdispersion polymerization, monomer is polymerized around core pigmentparticles in the presence of a steric stabilizer polymer soluble in thereaction medium. The solvent selected as the reaction medium must be agood solvent for both the monomer and the steric stabilizer polymer, buta non-solvent for the polymer shell being formed. For example, in analiphatic hydrocarbon solvent of Isopar G®, monomer methylmethacrylateis soluble; but after polymerization, the resultingpolymethylmethacrylate is not soluble.

The polymer shell formed from the monomer must be completelyincompatible or relatively incompatible with the solvent in which thecomposite pigment particles are dispersed. Suitable monomers may bethose described above, such as styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylateor the like.

The steric stabilizer polymer may be a reactive and polymerizablemacromonomer which adsorbs, becomes incorporated or is chemically bondedonto the surface of the polymer shell being formed. The macromonomer asa steric stabilizer, determines the particle size and colloidalstability of the system.

The macromonomer may be an acrylate-terminated or vinyl-terminatedmacromolecule, which are suitable because the acrylate or vinyl groupcan co-polymerize with the monomer in the reaction medium.

The macromonomer preferably has a long tail, R, which may stabilize thecomposite pigment particles in a hydrocarbon solvent.

One type of macromonomers which is suitable for the process is PE-PEOmacromonomers, as shown below:R_(m)O—[—CH₂CH₂O—]_(n)—CH₂-phenyl-CH═CH₂orR_(m)O—[—CH₂CH₂O—]_(n)—C(═O)—C(CH₂)═CH₂

The substituent R may be a polyethylene chain, n is 1-60 and m is 1-500.The synthesis of these compounds may be found in Dongri Chao et al.,Polymer Journal, Vol. 23, no. 9, 1045 (1991) and Koichi Ito et al,Macromolecules, 1991, 24, 2348.

Another type of suitable macromonomers is PE macromonomers, as shownbelow:CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The n, in this case, is 30-100. The synthesis of this type ofmacromonomers may be found in Seigou Kawaguchi et al, Designed Monomersand Polymers, 2000, 3, 263.

To incorporate functional groups for charge generation, a co-monomer maybe added in the reaction medium. The co-monomer may either directlycharge the composite pigment particles or have interaction with a chargecontrol agent in the display fluid to bring a desired charge polarityand charge density to the composite pigment particles. Suitableco-monomers may includevinylbenzylaminoethylamino-propyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid and the like.

Alternatively, the composite pigment particles may be prepared by livingradical dispersion polymerization, as shown in FIG. 2.

The living radical dispersion polymerization technique is similar to thedispersion polymerization described above by starting the process withinorganic pigment particles (21) and monomer dispersed in a reactionmedium.

The monomers used in the process to form the shell (22) may includestyrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butylmethacrylate, t-butyl acrylate, t-butyl methacrylate, vinyl pyridine,n-vinyl pyrrolidone, 2-hydoxyethyl acrylate, 2-hydroxyethylmethacrylate, dimethylaminoethyl methacrylate and the like.

However in this alternative process, multiple living ends (24) areformed on the surface of the shell (22). The living ends may be createdby adding an agent such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy),a RAFT (reversible addition-fragmentation chain transfer) reagent or thelike, in the reaction medium, for the living radical polymerization.

In a further step, a second monomer is added to the reaction medium tocause the living ends (24) to react with the second monomer to form thesteric stabilizers (23). The second monomer may be lauryl acrylate,lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate,hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octylmethacrylate, n-octadecyl acrylate, n-octadecyl methacrylate or thelike.

The steric stabilizers should be compatible with the solvent in whichthe composite pigment particles are dispersed to facilitate dispersionof the composite pigment particles in the solvent.

The steric stabilizers may also be prepared through living radicalpolymerization.

A co-monomer may also be added to generate charge. Suitable co-monomersmay include vinylbenzylaminoethylaminopropyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid and the like.

Further alternatively, the composite pigment particles may be formed bycoating core pigment particles with polyurethane and/or polyurea.

Polyurethane and polyurea usually are not compatible to a non-polarhydrocarbon solvent and their hardness and elastic property can be tunedthrough the monomer composition.

In the composite pigment particles of the present invention, the shellmay be a polyurethane or polyurea material. The steric stabilizers maybe non-polar long chain hydrocarbon molecules.

The synthesis method is similar to emulsion or dispersionpolymerization, except that polycondensation occurs, inside micelles,with polyurethane monomer and the inorganic core pigment particles.

The polyurethane or polyurea coating system may be considered as anoil-in-oil emulsion, which contains two incompatible solvents, one ofwhich is a non-polar organic solvent and the other is a polar organicsolvent. The system may also be referred to as non-aqueous emulsionpolycondensation, in which the non-polar solvent is the continuous phaseand the polar solvent is the non-continuous phase. The monomer and theinorganic pigment particles are in the non-continuous phase. Suitablenon-polar solvents may include the solvents in the Isopar® series,cyclohexane, tetradecane, hexane or the like. The polar solvents mayinclude acetonitrile, DMF and the like.

An emulsifier or dispersant is critical for this biphasic organicsystem. The molecular structure of the emulsifier or dispersant maycontain one part soluble in the non-polar solvent, and another partanchoring to the polar phase. This will stabilize the micelles/dropletscontaining the monomer and the inorganic pigment particles and servingas a micro-reactor for the particle formation through polycondensation.

Suitable emulsifiers or dispersants may include di-block co-polymers,such as poly(isoprene)-b-poly(methyl methacrylate),polystyrene-b-poly(ethene-alt-propene) (Kraton) or the like.

Also, a co-emulsifier may be added to form chemical bonding with theparticles. For example, amine terminated hydrocarbon molecules can reactwith the particles during polycondensation and bond to surface as robuststeric stabilizers. Suitable co-emulsifiers may include surfonamine(B-60, B-100 or B-200) as shown below:CH₃—[—OCH₂CH₂—]_(x)—[—OCH₂CH(CH₃)—]_(y)—NH₂wherein x is 5-40 and y is 1-40.

An alternative approach is to continue growing polyacrylate stericstabilizers after the polycondensation reaction in the microreactor iscompleted. In this case, the shell is formed from polyurethane while thesteric stabilizers may be polyacrylate chains. After the emulsifier ordispersant used in the process is washed away from the particle surface,the composite pigment particles are stable in the non-polar solvent(i.e., display fluid) with the polyacrylate stabilizers. Some materialsthat can initiate acrylate polymerization include isocyanatoethylacrylate, isocyanatostyrene or the like.

Monomers for the steric stabilizer may be a mixture of hydroxyethylmethacrylate and other acrylate that are compatible to the non-polarsolvent, such as lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate,n-octadecyl methacrylate or the like.

In any of the processes described above, the quantities of the reagentsused (e.g., the inorganic core pigment particles, the shell material andthe material for forming the steric stabilizers) may be adjusted andcontrolled to achieve the desired organic content in the resultingcomposite pigment particles.

The third aspect of the present invention is directed to a display fluidcomprising the composite pigment particles of the present invention,which composite pigment particles are dispersed in a solvent. Apreferred solvent has a low dielectric constant (preferably about 2 to3), a high volume resistivity (preferably about 1015 ohm-cm or higher)and a low water solubility (preferably less than 10 parts per million).Suitable hydrocarbon solvents may include, but are not limited to,dodecane, tetradecane, the aliphatic hydrocarbons in the Isopar® series(Exxon, Houston, Tex.) and the like. The solvent can also be a mixtureof a hydrocarbon and a halogenated carbon or silicone oil base material.

The present invention is applicable to a one-particle or two-particleelectrophoretic display fluid system.

In other words, the present invention may be directed to a display fluidcomprising only the composite pigment particles prepared according tothe present invention which are dispersed in a hydrocarbon solvent. Thecomposite pigment particles and the solvent have contrasting colors.

Alternatively, the present invention may be directed to a display fluidcomprising two types of pigment particles dispersed in an organicsolvent and at least one of the two types of the pigment particles isprepared according to the present invention. The two types of pigmentparticles carry opposite charge polarities and have contrasting colors.For example, the two types of pigment particles may be black and whiterespectively. In this case, the black particles may be preparedaccording to the present invention, or the white particles may beprepared according to the present invention, or both black and whiteparticles may be prepared according to the present invention.

The composite pigment particles prepared according to the presentinvention, when dispersed in an organic solvent, have many advantages.For example, the density of the composite pigment particles may besubstantially matched to the organic solvent, thus improving performanceof the display device. In other words, the difference between thedensity of the composite pigment particles and the density of thesolvent is less than 2 g/cm³, more preferably less than 1.5 g/cm³ andmost preferably less than 1 g/cm³.

In the two particle system, if only one type of the pigment particles isprepared according to the present invention, the other type of pigmentparticles may be prepared by any other methods. For example, theparticles may be polymer encapsulated pigment particles.Microencapsulation of the pigment particles may be accomplishedchemically or physically. Typical microencapsulation processes includeinterfacial polymerization/crosslinking, in-situpolymerization/crosslinking, phase separation, simple or complexcoacervation, electrostatic coating, spray drying, fluidized bed coatingand solvent evaporation, etc.

The pigment particles prepared by the previously known techniques mayalso exhibit a natural charge, or may be charged explicitly using acharge control agent, or may acquire a charge when suspended in theorganic solvent. Suitable charge control agents are well known in theart; they may be polymeric or non-polymeric in nature, and may also beionic or non-ionic, including ionic surfactants such as dye materials,sodium dodecylbenzenesulfonate, metal soap, polybutene succinimide,maleic anhydride copolymers, vinylpyridine copolymers, vinylpyrrolidonecopolymer, (meth)acrylic acid copolymers, N,N-dimethylaminoethyl(meth)acrylate copolymers or the like.

EXAMPLE Step A: Deposition ofVinylbenzylaminoethylaminopropyl-trimethoxysilane on Black PigmentParticles

To a 1 L reactor, Black 444 (Shepherd, 40 g), isopropanol (320 g), DIwater (12 g), ammonium hydroxide (28%, 0.4 g) and Z-6032 (Dow Corning,16 g, 40% in methanol) were added. The reactor was heated to 65° C. withmechanical stirring in a sonication bath. After 5 hours, the mixture wascentrifuged at 6000 rpm for 10 minutes. The solids were redispersed inisopropanol (300 g), centrifuged and dried at 50° C. under vacuumovernight to produce 38 g of desired pigment particles.

Step B: Preparation of Polymer Coating on Pigment Particles throughDispersion Polymerization

Two (2) g of polyvinylpyrrolidone (PVP K30) was dissolved in a mixtureof 94.5 g water and 5.5 g ethanol. The solution was purged with nitrogenfor 20 minutes and heated to 65° C. The pigment particles (4 g) preparedfrom Step A was dispersed in a mixture of 3.0 g lauryl acrylate, 0.2 gdivinylbezene and 0.03 g AIBN (azobisisobutyronitrile) to form a uniformsuspension. This suspension was added into the PVP solution at 65° C.With stirring, the polymerization reaction lasted about 12 hours.

Then a mixture of 3.0 g octadecyl acrylate and 0.03 g AIBN was addedinto the above reaction flask and the reaction was continued for 12hours.

The solids produced were separated from the liquid throughcentrifugation and then washed with isopropanol and methylethylketone toremove PVP K30 and other chemicals that were not bonded on the pigmentparticles. The solids were dried at 50° C. under vacuum to produce finalcomposite black particles. The organic content of the particles producedwas about 34% by weight, tested through TGA (thermal gravimetricanalysis).

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention.

What is claimed is:
 1. A display fluid comprising a charge control agentand charged composite pigment particles dispersed in a hydrocarbonsolvent, wherein each of said composite pigment particles consisting ofat least a core pigment particle, an organic polymer shell, and stericstabilizer polymers, wherein each of the steric stabilizer polymers ischemically bonded on the surface of the organic polymer shell, the totalweight of the shell and the steric stabilizer polymers in the totalweight of the core pigment particle, the shell, and the stericstabilizer polymers is 20-70% by weight, the display fluid is anelectrophoretic fluid, and the steric stabilizer polymers are selectedfrom the group consisting of: polyethylene, polypropylene, polyester,polysiloxane, and a mixture thereof.
 2. The fluid of claim 1, whereinthe density of the composite pigment particles substantially matches tothat of the solvent.
 3. The fluid of claim 2, wherein the differencebetween the density of the composite pigment particles and the densityof the solvent is less than 2 g/cm³.
 4. The fluid of claim 1, whereinsaid core pigment particle is an inorganic pigment particle.
 5. Thefluid of claim 1, wherein said core pigment particles are surfacetreated.
 6. The fluid of claim 1, wherein said organic shell is formedfrom polyacrylate, polyurethane, polyurea, polyethylene, polyester orpolysiloxane.
 7. The fluid of claim 1, wherein the total weight of theshell and the steric stabilizer polymers in the total weight of the corepigment particle, the shell, and the steric stabilizer polymers is20-40% by weight.
 8. The fluid of claim 1, wherein said shell iscompletely incompatible or relatively incompatible with the solvent. 9.The fluid of claim 1, wherein the surface of the organic shell comprisesfunctional groups to enable charge generation or interaction with thecharge control agent.
 10. The fluid of claim 1, further comprising asecond type of charged pigment particles.
 11. The fluid of claim 10,wherein each of said second type of charged pigment particles consistingof at least a core pigment particle, an organic polymer shell, andsteric stabilizer polymers, wherein each of the steric stabilizerpolymers is chemically bonded on the surface of the organic polymershell; and the two types of charged pigment particles are of contrastingcolors.
 12. The fluid of claim 10, wherein each of said second type ofcharged pigment particles is prepared by a conventionalmicroencapsulation technique.
 13. The fluid of claim 1, wherein saidcomposite pigment particles are prepared by dispersion polymerization.14. The fluid of claim 1, wherein said composite pigment particles areprepared by living radical polymerization.
 15. The fluid of claim 8,wherein no more than 5% of said shell is miscible with the displayfluid.
 16. The fluid of claim 15, wherein no more than 1% of said shellis miscible with the display fluid.
 17. The fluid of claim 1, whereinsaid shell is formed from a polymer material having a polarfunctionality on a main chain or a side chain.
 18. The fluid of claim17, wherein the polar functionality is —COOH, —OH, —NH₂, R—O—R orR—NH—R, wherein R is an alkyl or aryl group.
 19. The fluid of claim 17,wherein the side chain has an aromatic moiety.
 20. The fluid of claim 1,wherein the core pigment particle and the shell are a single unit. 21.The fluid of claim 1, wherein the organic shell is formed from monomersselected from the group consisting of styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate and dimethylaminoethylmethacrylate.